1. Field of the Invention
The invention relates to a continuous process for the preparation of aryl carbonates from carbonates containing at least one aliphatic ester group and phenols on the one hand and from alkyl aryl carbonates on the other hand by catalysed transesterification, the reaction being carried out in one or more bubble columns.
2. Description of the Related Art
The preparation of aromatic and aliphatic-aromatic carbonic esters (carbonates) by transesterification, starting from aliphatic carbonic esters and phenols, is known in principle. This is an equilibrium reaction, the position of the equilibrium being shifted almost completely in the direction of the aliphatically substituted carbonates. Therefore, it is relatively easy to prepare aliphatic carbonates from aromatic carbonates and alcohols. However, in order to carry out the reaction in the reverse direction towards aromatic carbonates, it is necessary to shift effectively the highly unfavourably lying equilibrium, not only highly active catalysts, but also a favourable procedure having to be used.
For the transesterification of aliphatic carbonic esters with phenols, a multiplicity of effective catalysts have been recommended, such as for example alkali metal hydroxides, Lewis acid catalysts selected from the group comprising the metal halides (German Offenlegungsschrift 2 528 412 and 2 552 907), organotin compounds (EP 0 000 879, EP 0 000 880, German Offenlegungsschrift 3 445 552, EP 0 338 760), lead compounds (JP 57/176 932), Lewis acid/proton acid catalysts (German Offenlegungsschrift 3 445 553).
In the known processes, the transesterification is carried out in a batchwise reactor at atmospheric pressure or under pressure, with or without an additional separation column. Even with the most highly active catalysts, reaction times of many hours are required in these cases to achieve even only average conversion rates of approximately 50% of phenol. Thus in the batchwise transesterification of phenol with diethyl carbonate at 180.degree. C. using various organotin compounds, as described in German Offenlegungsschrift 3 445 552, yields of diphenyl carbonate of an order of magnitude of more than 20% are only achieved after a reaction time of approximately 24 hours; in the batchwise transesterification of phenol and dimethyl carbonate with the aid of organotin catalysts, as described in EP 0 000 879, the phenol conversion rate after 30 h is 34% of the theoretical value.
This means that, owing to the unfavourable thermodynamic conditions, the batchwise transesterification reactions described, even with the use of highly active catalyst systems, can only be carried out in the sense of an industrial process highly disadvantageously, since very poor space-time yields and high residence times with high reaction temperatures are required.
Such procedures are also particularly disadvantageous since even with highly selective transesterification catalysts at high temperatures and with long residence times of many hours, a marked proportion of side reactions occurs, for example ether formation with elimination of carbon dioxide.
It was therefore attempted to shift the reaction equilibrium as rapidly as possible in the direction of the products by adsorption to molecular sieves of the alcohol resulting in the transesterification (German Offenlegungsschrift 3 308 921). From the description of this procedure it appears that, for the adsorption of the reaction alcohol, a large amount of molecular sieve is required, which exceeds the amount of liberated alcohol by at least five fold. Furthermore, the molecular sieves used must be regenerated even after a short time and the conversion rate to the alkyl aryl carbonate intermediates is relatively low. This process therefore also appears not to be advantageously industrially and economically applicable.
A continuous transesterification process for the preparation of aromatic carbonates in which the reaction is carried out in one or more multiple-stage sequentially-connected distillation columns is described in EP-A 0 461 274. In this case, phenols are initially reacted with dialkyl carbonates to give aryl carbonate mixtures which in the main contain alkyl aryl carbonates. In a second, preferably downstream, multiple-stage distillation column, these are then further reacted to give the desired diaryl carbonate end products. The applicant emphasizes the effectiveness and the selectivity of its procedure.
Apart from conversion rates and selectivity, the citation of the space-time yield (STY) serves as a criterion for the evaluation of a process for those skilled in the art, since it describes the yield of product per unit of apparatus volume used. By way of the example of the transesterification of dimethyl carbonate (DMC) with phenol to give methyl phenyl carbonate (MPC) and diphenyl carbonate (DPC), the applicant of EP 0 461 274 shows a comparison of the batch mode of operation in an autoclave (Comparative Example 1) with a mode of operation in a multiple-stage distillation column (Example 1). In this case, only an increase of the STY from 5 to 8 g of the sum of DPC+MPC/1.h is achieved, as can easily be calculated from the examples. The STYs are comparatively low in both examples; only the MPC selectivity increased in the mode of operation in a multiple-stage distillation column from 94% to 97%. These results are achieved already under optimal conditions with the best transesterification catalysts at high temperatures and elevated pressure, so that further improvements do not appear to be possible.
The further reaction of the alkyl aryl carbonates to give diaryl carbonates proceeds in the procedure cited, as follows from the examples, in the sense of a disproportionation reaction. It is thus no wonder that in this reaction proceeding more readily in comparison to the first transesterification stage significantly higher STYs are achieved.
For the second transesterification stage, EP 0 461 274 compares the transesterification of methyl phenyl carbonate (MPC) to give diphenyl carbonate (DPC) in the batch mode of operation in the autoclave (Comparative Example 2) with carrying out the transesterification in a multiple-stage distillation column (Example 11). In this case, the STYs for DPC calculated from the data given there even show a reduction in the effectiveness from 144 g of DPC/1.h to 133 g of DPC/1.h. Only the formation of the by-product anisole occurs to a lesser extent.
Because of these figures and the considerably higher apparatus complexity, the improvement demonstrated here must be evaluated extremely sceptically.
The aim of an improvement of the transesterification reaction according to the invention should therefore primarily be an increase of the STYs, primarily of the transesterification stages with phenol, in which the selectivity of the overall process should not be reduced.